1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium comprising an electron-donating coloring compound (hereinafter referred to as a coloring agent) and an electron-accepting compound (hereinafter referred to as a color developer), capable of inducing color formation by utilizing the coloring reaction between the electron-donating compound and the electron-accepting compound. More specifically, the present invention relates to a reversible thermosensitive recording medium capable of forming a colored image therein and erasing the same therefrom by controlling the thermal energy applied thereto.
2. Discussion of Background
There is conventionally known a thermosensitive recording medium utilizing the coloring reaction between the electron-donating compound (or the coloring agent) and the electron-accepting compound (or the color developer). This kind of thermosensitive recording medium is widely used, for example, for a facsimile apparatus, a word processor, and a printer of a scientific measuring instrument. However, the coloring reaction of the conventional thermosensitive recording medium of this type has no reversibility, so that color development and decolorization cannot be alternately repeated.
Among published patents, there are several proposals for a thermosensitive recording medium which can reversibly carry out the color development and decolorization utilizing a coloring reaction between a coloring agent and a color developer. For example, a thermosensitive recording medium using phloroglucinol and gallic acid as color developers in combination is disclosed in Japanese Laid-Open Patent Application 60-193691. There is disclosed in Japanese Laid-Open Patent Application 61-237684 a reversible thermosensitive recording medium which employs a compound such as phenolphthalein or thymolphthalein as the color developer. In Japanese Laid-Open Patent Applications 62-138556, 62-138568 and 62-140881, there are disclosed reversible thermosensitive recording media, each comprising a recording layer which contains a homogeneously dissolved composition of a coloring agent, a color developer and a carboxylic acid ester. Further, in Japanese Laid-Open Patent Application 63-173684, a reversible thermosensitive recording medium comprising as a color developer an ascorbic acid derivative is disclosed. In addition, a reversible thermosensitive recording medium comprising as a color developer a salt of bis(hydroxyphenyl)acetic acid or gallic acid, and a higher aliphatic amine is disclosed in Japanese Laid-Open Patent Applications 2-188293 and 2-188294.
In the above-mentioned conventional reversible thermosensitive recording media, however, both of the stability of a colored image and the facility for decolorization are not always satisfied at the same time, and there is the problem with respect to the density of a colored image, and the stability of the repeated coloring and decolorization operations. Therefore, the conventional reversible recording media are not satisfactory for practical use.
The inventors of the present invention have previously proposed a reversible thermosensitive coloring composition which comprises an organic phosphoric acid compound, an aliphatic carboxylic acid compound or a phenol compound, each having a long-chain aliphatic hydrocarbon group therein, as a color developer, and a leuco dye as a coloring agent, as disclosed in Japanese Laid-Open Patent Application 5-124360. By use of such a reversible thermosensitive coloring composition, the color development and the decolorization can be easily carried out by controlling the heating and cooling conditions for the coloring composition, and further, the color-developed state and the decolorized state can be stably maintained at room temperature, and the color development and the decolorization can be alternately repeated in a stable condition. In this application, there is also proposed a reversible thermosensitive recording medium which comprises a recording layer containing the above-mentioned reversible thermosensitive coloring composition.
Although this kind of reversible thermosensitive recording medium attains a satisfactory level for practical use with respect to the compatibility of the stability of a colored image with the facility for decolorization, and the density of a colored image, there is yet room for improvement of the adaptability to a wide range of operating conditions and the scope of application in terms of the color development and decolorization conditions. To be more specific, when the image portion and the background portion are exposed to the sunlight or fluorescent light for a long period of time, those portions are easily subjected to color change. In particular, the recorded image portion cannot be completely erased from the recording layer, with leaving a residual image even after the decolorization operation is carried out.
Further, there is the problem of moisture resistance stability that the image density of the recorded image portion is decreased during the storage under the circumstance of high humidity.
In Japanese Laid-Open Patent Application 6-210954, it is proposed to use a phenol compound having a long-chain aliphatic hydrocarbon group as the color developer. However, the above-mentioned problems of the light and moisture resistance stabilities cannot be solved by the reversible thermosensitive recording medium comprising the above-mentioned phenol compound as the color developer.
Furthermore, in order to improve the preservation stability of the colored image in terms of light resistance, it is proposed to overlay on the reversible thermosensitive recording layer a protective layer having a reflectance of less than 50% with respect to the ultraviolet light with a wavelength of 350 nm, as disclosed in Japanese Laid-Open Patent Application 7-205547. There is disclosed in the Examples of this application the method of forming a protective layer using a formulation which comprises microcapsules containing an organic ultraviolet absorbing agent. By providing the protective layer on the recording layer in the above-mentioned manner, however, the problem of the light resistance stability cannot be completely solved.